Melting a ferrous ion containing ferrimagnetic oxide in a ferric ion crucible



United States Patent Ofiic e 3,404,966 MELTING A FERROUS ION CONTAINING FERRIMAGNETIC OXIDE IN A FERRIC ION CRUCIBLE 1 William 1). Westwood and Derek C. Lewis, Ottawa, On-

tario, Canada, assignors to Northern Electric Company Limited, Montreal, Quebec,'-Canada No Drawing. Filed Sept. 4, 1964, Ser. No. 394,608 3 Claims. (Cl. 23-293) This invention relates to a crucible and a method for containing a substance to be melted by induction heating. The invention is particularly suitable for containing a melt of ferrimagnetic material during crystal growth from the melt by the Czochralski crystal pulling method.

For some time, it has been diflicult to obtain pure, uncontaminated melts of certain substances, particularly of compounds such as ferrimagnetic oxides. The difiiculties have existed because such substances have very high melting points and because there is a requirement for maintaining oxidizing atmospheres around the compounds during the melting to preserve the composition.

If a metal crucible is used for containing the ferrimagnetic substance to be melted and the crucible is heated by induction techniques, the substance in the crucible is heated by heat conduction from the crucible rather than directly. Since in this case the metal of the crucible is always at a higher temperature than the substance contained therein, melting of the crucible can occur. In addition, it the substance in the crucible is an oxide, undesirable oxidation in the crucible can occur.

If a non-conducting high melting point oxide which is not heated inductively, such as alumina, magnesia, or beryllia, is used as a crucible for containing the ferrimagnetic substance to be melted, the crucible would be dissolved by the ferrimagnetic melt.

From the above it can be seen that the employment of a metal or a high melting point oxide as the crucible for the ferrimagnetic substance to be melted results in contamination of the melt.

Furthermore, floating zone refining methods in which the substance holds its own melt by surface tension, are limited to solid shapes of materials in which a homogeneous molten zone can be supported.

Applicants have discovered that the above difliculties can be overcome by having the crucible made of material having substantially the same composition as the substance to be melted, but having a substantially higher resistivity than the substance.

According to applicants invention, a method for melting a ferrimagnetic oxide compound is provided which comprises the steps of placing the compound to be melted in a crucible having substantially the same composition as the compound to be melted, but having a substantially higher resistivity than the compound to be melted. Then, the compound is melted in the crucible by induction heating.

When the crucible containing the substance to be melted is placed in the presence of the high frequency electromagnetic field of an induction coil, the crucible is not heated inductively because of its very high resistivity and is only heated magnetically in the high frequency coil. This magnetic heating is due to the hysteresis losses in the ferrimagnetic material but these losses become negligible when the material becomes paramagnetic near its Curie temperature and the heating of the crucible ceases. However, some heating of the crucible is desirable since it helps to prevent thermal shock to the crucible which might otherwise fracture it. The low resistivity substance, on the other hand, may be brought to its melting point by induction heating. The low resistivity substance can be heated in the presence of oxygen at temperatures near 3,404,966 Patented Oct. 8, 1968 the melting point of the substance causing oxidation of the substance with a consequent increase in its resistivity, thereby causing the inductive heating to cease.

Thus, a crucible made of high resistivity material can contain low resistivity material of the same composition (apart from any oxygen content), so that there is no possibility of contamination of the melt. The amount of oxygen in the melt may be controlled by the partial pressure of oxygen above the melt.

One ferrimagnetic oxide that has been successfully contained as a melt in accordance with this invention is yttrium iron garnet. It is well known that pure yttrium iron garnet has a very high resistivity. The iron in the high resistivity form of this compound is in the form of ferric ion. As applicants have disclosed in their copending application Ser. No. 394,603, filed concurrently with this application, now abandoned in favour of continuation application Ser. No. 662,559, filed Aug. 2, 1967, the resistivity of a ferrimagnetic compound can be greatly reduced by partially reducing the ferric ion content of the material to ferrous ion. The teachings of this copending application are hereby incoporated by reference.

The crucible may be constructed from toroidal and cylindrical shapes manufactured by known ceramic techniques with the preparation conditions controlled so that the resistivity of the material is very high and its porosity low. In the case of yttrium iron garnet, this has been achieved by calcining the mixed oxides twice at about 1200 C. and pressing at 20,000 p.s.i. to form the shapes. The flat edges of the shapes may then be slightly roughened, coated with a slurry of the mixed oxides and pressed together in the form of a crucible. The form is then fired in an oxygen atmosphere at about 1450 C. which binds the form together into a crucible. Alternatively, the crucibles may be made in a single step by any suitable fabrication technique, e.g., pressing, slip casting, etc.

In this state, the yttrium iron garnet material has a very high resistivity, greater than 10 ohm-centimeters. Thus very little heating of the crucible takes place when it is placed within the high frequency induction coil. For the substance to be melted, a pellet of yttrium iron garnet is then prepared by known ceramic techniques and is then fired in a reducing atmosphere such as hydrogen or nitrogen to obtain conversion of some ferric ions to ferrous ions in the manner taught by the above-mentioned copending application. These pellets so treated result in a substance of substantially decreased resistivity which may be less than ohm-centimeters.

The pellets are then placed in the crucible and are readily coupled to a high frequency induction coil where melting of the pellets takes place by induction heating. During experiments, applicants coupled these pellets to a four-turn coil attached to the 3 me. terminals of a 6 kw. Lepel generator.

When the pellets are fully melted, powder of the same material can be added to the melt as required.

Applicants prefer to first fill the crucible with powder of the substance to be melted, and then place the pellet on top of the powder. If this melt is to be used for crystal growth, then any composition of powder can be used which enables variation of the melt composition to determine the best conditions for growth.

Thus, according to their invention, applicants have provided a new crucible and method which permits a ferrimagnetic oxide compound to be melted to be contained without deterioration to the crucible which would lead to contamination of the melt.

What is claimed is:

1. A method for melting a ferrimagnetic oxide compound in a crucible by induction heating without the compound being contaminated by the crucibletcomprisp ofa W (a) placing a pellet of the ferrimagnetic oxide compound to be melted in a crucible formed of the same ferrimagnetic oxide compound, the pellet and the crucible having substantially the same composition except that iron in the crucible is substantially only in the form of ferric ion, whereas at least a portion of the iron in the pellet is in the form of ferrous ion and is present in an amount to reduce the resistivity of the substance to melting,

(b) and melting the pellet in the crucible by induction heating, whereby due to its ferric ion content and consequent higher resistivity, the crucible is heated an amount insufficient to cause its deterioration.

2. A method for melting a ferrimagnetic oxide compound in a crucible by induction heating without the compound being contaminated by the crucible, comprising the steps of:

(a) constructing a crucible formed of a ferrimagnetic oxide compound in which the iron is substantially only in the form of ferric ion;

(b) preparing a pellet of the ferrimagnetic compound to be melted, the pellet having substantially the same composition as the crucible;

(c) firing the pellet in a reducing atmosphere to convert at least a portion of the ferric ion content of the pellet to ferrous ion whereby the resistivity of the pellet is reduced;

I (d) placing the pellet of reduced resistivity in the crucible; s I s (e) and melting the pellet of reduced resistivity in the crucible by induction heating, whereby due to its ferric ion content and consequent higher resistivity, the crucible is heated an amount insufficient to cause its deterioration.

3. A method as defined in claim 2 wherein the compound is yttrium iron garnet, the resistivity of the crucible is greater than 1X 10 ohm-centimeters and the resistivity of the pellet is less than 100 ohm-centimeters.

References Cited UNITED STATES PATENTS 2,743,199 4/1956 Hull 23-30l 2,904,512 9/1959 Horn 23-301 2,927,896 3/1960 Bergmann 25262.5 3,050,407 8/1962 Nielsen 25262.5 3,051,555 8/1962 Rummel 23301 3,079,240 2/1963 Remeika 25262.5 3,085,980 4/1963 Gortev 25262.5 3,090,673 5/1963 Sterling 23301 3,258,314 6/1966 Redmond 2330l NORMAN YUDKOFF, Primary Examiner.

G. P. HINES, Assistant Examiner. 

1. A METHOD FOR MELTING A FERRIMAGNETIC OXIDE COMPOUND IN A CRUCIBLE BY INDUCTION HEATING WITHOUT THE COMPOUND BEING CONTAMINATED BY THE CRUCIBLE, COMPRISING THE STEPS OF: (A) PLACING A PELLET OF THE FERRIMAGNETIC OXIDE COMPOUND TO BE MELTED IN A CRUCIBLE FORMED OF THE SAME FERRIMAGNETIC OXIDE COMPOUND, THE PELLET AND THE CRUCIBLE HAVING SUBSTANTIALLY THE SAME COMPOSITION EXCEPT THAT IRON IN THE CRUCIBLE IS SUBSTANTIALLY ONLY IN THE FORM OF FERRIC ION, WHEREAS AT LEAST A PORTION OF THE IRON IN THE PELLET IS IN THE FORM OF FERROUS ION AND IS PRESENT IN AN AMOUNT TO REDUCE THE RESISTIVITY OF THE SUBSTANCE TO MELTING, (B) AND MELTING THE PELLET IN THE CRUCIBLE BY INDUCT ON HEATING, WHEREBY DUE TO ITS FERRIC ION CONTENT AND CONSEQUENT HIGHER RESISTIVITY, THE CRUCIBLE IS HEATED AN AMOUNT INSUFFICIENT TO CAUSE ITS DETERIORATION. 